I would like to operate the PLH at 28V, as I have 28V,4A regulated power supply. What are the changes needed in the circuit. Also I would like to operate this amp only in push-pull mode. Can I replace P2 with 390ohms + 110 ohms series connected resistors with the bootstrap capacitor connected at their junction?

I don't recall testing the amp at 28 volts, but probably
you will be able to adjust P1 through P3 to get what you
want. If not, you will need to revise the values of the
resistors in series with those pots. R4 and R6 would go
down in value, and R7 would go up.